Method and structure for automated inert gas charging in a reticle stocker

ABSTRACT

At least a first reticle is stored in a housing of a stocker. A first gas is delivered to the housing. At least one reticle pod having an additional reticle is delivered into a enclosure within the housing of the stocker. A second gas different from the first gas is delivered to the enclosure. The reticle pod is automatically retrieved from the enclosure. The delivery and retrieval of the reticle pod and delivery of the first gas and the second gas are automatically controlled.

FIELD OF THE INVENTION

The present invention relates to control systems generally, and morespecifically to a control method and system for stockers in an automatedmaterial handling system (AMHS).

BACKGROUND

AMHS's have been used extensively in the semiconductor fabricationfield. The typical system includes a plurality of bays (rows) of storageareas. Each bay has a stocker, which includes bins for holding aplurality of containers, such as standard mechanical interface (SMIF)containers for loading 200 mm (8 inch) wafers, or front opening unifiedpods (FOUPs), which may be used to load 300 mm (12 inch) wafers. Thestocker holds the SMIFs or FOUPs in preparation for transporting a SMIFor FOUP to the loadport of a processing tool. In technologies usinglarger critical dimensions (CD) and 200 mm wafers, a reticle stockerstored and delivered reticles for use in photolithographic processes. Anoverhead hoist transport (OHT) associated with each bay transports theSMIF or FOUP with wafers, or the pod with a reticle, from the respectivestocker to a loadport for processing in one of the tools (fabricationprocess machines).

In conventional technologies using reticles with larger CDs, it wascommon to cover each reticle with a pellicle. The pellicle was a thinfilm of polymer secured to the reticle to prevent dirt from collectingdirectly on the reticle. The pellicle kept any foreign particles outsideof the focal plane of the photolithographic tool used for patterning thewafer. For higher technology fabrication processes (90 nm, 65 nm andtechnologies with smaller critical dimensions), the pellicle material isnot transparent to the extreme ultra-violet (EUV) radiation used.Because of this, higher technology reticles are often provided without apellicle. Without the protection of the pellicle, other means are usedto protect the reticle from contamination and precipitation of foreignsubstances.

It is known that charging (exposing) the reticle in a nitrogen (N₂)atmosphere immediately before and immediately after the reticle is usedfor patterning a wafer will purge the reticle of impurities. Theimpurities diffuse out of the reticle material into the purging gas.

Thus, as shown in FIG. 1, the higher technology (smaller CD) reticles201 were stored in a separate cabinet 202 and manually delivered by aworker 203 to an N₂ charging tool 204 before and after use in thefabrication tool 205. A semiconductor fabrication facility (“fab”) mayhave many low volume density N₂ cabinets 202 that consume significantspace. These cabinets 202 were hard to manage because they were indispersed locations. The handling procedure for reticle pod N₂ chargingby the operator was also complicated. This impacted the operationefficiency of the tool 205. In some instances, delay occurred when a lot206 of wafers was ready for processing by the tool 205, but the N₂purged reticle 201 was not yet available at the tool. Errors intransferring the reticles 201 to and from the N₂ cabinet 202 and failingto properly perform the N₂ purge operation caused high reticle repairingrates and lots were often scrapped due to precipitation of materials onreticles 201 that were used to process those lots.

SUMMARY

A method for storing and delivering a reticle comprising storing atleast a first reticle in a housing of a stocker. A first gas isdelivered to the housing. At least one reticle pod having an additionalreticle is automatically delivered into an enclosure having a separateambient environment within the housing of the stocker. A second gasdifferent from the first gas is delivered to the enclosure. The reticlepod is automatically retrieved from the enclosure. Delivery andretrieval of the reticle pod and delivery of the first gas and thesecond gas are automatically controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a conventional process for retrieving areticle pod from a storage cabinet, performing an N₂ charging operation,and delivering the reticle to a tool.

FIG. 2 is a block diagram of an exemplary embodiment of an automatedreticle stocker.

FIG. 3 is another block diagram of the system of FIG. 2, showing thecontrol components.

FIG. 4 is a flow chart of operation of the control elements shown inFIG. 3.

FIG. 5A is a partial cutaway isometric view of the stocker shown inFIGS. 2 and 3.

FIG. 5B is an enlarged detail of the enclosure (purge nest) of FIGS. 2and 3.

FIG. 6 is a flow chart of an exemplary method of automaticallycontrolling and operating the reticle stocker.

FIG. 7 is a flow chart showing a method of assigning priority for purgeoperations performed in the reticle stocker.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read inconnection with the accompanying drawings, which are to be consideredpart of the entire written description. Terms concerning attachments,coupling and the like, such as “connected” and “interconnected,” referto a relationship wherein structures are secured or attached to oneanother either directly or indirectly through intervening structures, aswell as both movable or rigid attachments or relationships, unlessexpressly described otherwise.

Because the availability of reticles to process wafers at the time theequipment is ready to perform the processing impacts on the overallproduction rate, it is desirable to operate the AMHS in a manner thatsupplies reticles quickly as soon as they are needed.

A solution is described herein suitable for storage and haze preventionof advanced reticles, such as those used for 300 mm wafers, 450 mmwafers, and the like. The reticle stocker provides high reticle PODstorage capacity for a phase shifting mask (PSM) that is required to bepreserved in a specific environment by charging the reticles with aninert gas such N₂ or purified clean dry air (XCDA).

FIG. 1 shows the conventional process for charging reticles. Reticles201 were stored in an N₂ cabinet 202. Then, a worker 203 would remove areticle 201 and place it in an N₂ charge tool 204, which would chargethe reticle. The reticle 201 was than moved to a scanner tool 205. Oftenthis sequence would cause delays for waiting lots 206.

As shown in FIG. 2, instead of manually handling the reticle pods 2, thesystem 4 offers the function to forecast incoming work-in-progress (WIP)and automatically charge the reticle 8 with inert gas (e.g., N₂) withinthe reticle pod 2 both before and after using the reticle 8 for aphotolithographic process.

One example is described below in which a reticle 8 may be stored withinan automated stocker 10 in a first gas such as clean dry air (CDA) orpurified CDA (XCDA), and the reticle 8, within its pod 2, can beautomatically delivered to a purge nest 12 (an enclosure, within thehousing 14 of the stocker 10, having a separate ambient environment fromthe rest of the stocker housing 14). The purge nest 12 has an N₂delivery mechanism 16 (shown in FIG. 5A) for performing the chargingoperation to perform a nitrogen purge of contaminants from the reticle8. One of ordinary skill in the art understands that a variety of firstgases (other than CDA) may be used throughout the stocker 10, and avariety of second gases (other than N₂) may be used within the purgenest 12, for handling reticles 8 with different storage and processingrequirements.

FIGS. 2, 3 and 5A show an exemplary reticle stocker 10 for storingsemiconductor fabrication reticles 8. The stocker 10 includes a stockerhousing 14. At least one receptacle 15 is provided within the stockerhousing 14 for storing at least one reticle 8. Preferably a plurality ofreceptacles are configured in the housing 14 for containing reticles 8,or for containing reticle pods 2 that in turn contain reticles 8.

As best seen in FIG. 5A, the reticle stocker 10 includes a means fordelivering a first gas to the at least one receptacle 2. In someembodiments, the first gas may be CDA or XCDA. The means for deliveringthe first gas may include a CDA (or XCDA) supply conduit 18. In someembodiments, the first gas is provided continuously into the housing 14,so that no control valves are needed within or near the housing 14 forthe first gas. In other embodiments, a control valve 20 one-way may beprovided in the stocker 10 for the first gas. The inlet gas should bepassed through a chemical filter 22, and should be a downward flow, sothat any foreign particles dislodged by the first gas fall down to thebottom of the stocker 14 for removal, and are not recirculated withinthe stocker 14. For example, the first gas inlet 24 may be placed at ornear a top of the housing 14, and a gas outlet 26 may be near the bottomof the housing 14. Pressure in the stocker 14 is desired to be higherthan outside ambient by at least 10 mm H₂O. The charging connectors 28are preferably particle proof or at least not susceptible to damage fromexpected particle impact.

In alternative embodiments, where the stocker 10 is placed in a CDA (orXCDA) clean room environment, the housing 14 of the stocker 10 mayinclude one or more openings to permit ingress and egress of the ambientCDA (or XCDA) to deliver the first gas.

In some embodiments, the stocker 10 has a sealable enclosure 12 withinthe stocker housing 14 for receiving at least one reticle pod 2 havingan additional reticle 42 therein. For example, the sealable enclosure 12may be a purge nest with closable valves.

In some embodiments, the purge nest 12, as shown in FIG. 5B, may includea plurality of vertically adjustable stages 502 for holding pods 2 ofvarious sizes. For example, by raising the stage 502 of layer 2 by thedistance 514, the inlet port and outlet port of pod 2 is brought intoengagement with the charging port 506 and exhaust port 507 of the purgenest 12 for performing a purge operation. Raising and/or lowering thestages 502 of each layer also increase the distance 512 between layersto ensure adequate clearance during positioning, and good gascirculation during the purge operation. Each stage 502 may be supportedby guide cylinders 510. A robot arm 508 may be provided to handle andposition the pod 2 within the purge nest 12. Rotary clamp cylinders 504may be provided to tightly close the pod 2 to ensure proper sealing.

In some embodiments, the purge nest 12 provides a charging port 506designed to fit properly with the inlet port (not shown) of the reticlepod 2. The reticle pod 2 is connected to the charging port 506 and thepurging gas (e.g., N₂) is delivered to the reticle pod 2. The chargingport 506 may have a check valve (not shown) to prevent egress of gasfrom the reticle pod 2 back into the N₂ supply. The reticle pod 2 alsohas an outlet port 36, from which the N₂ can escape into the purge nest12 by way of the exhaust port 507 of the purge nest 12. The purge nest12 may further include another one-way check valve 38 for its outputport 36, to prevent the ambient first gas (CDA or XCDA) in the housing14 of the stocker 10 from entering the purge nest 12.

The stocker 10 has a means for delivering a second gas (such as N₂, XCDAor an inert gas) to the enclosure (purge nest) 12. For example, themeans for delivering the second gas may include a pipe 44 that deliversthe second gas, under control of a flow control valve (not shown) thatis controlled by an N₂ controller 46.

As best seen in FIG. 5A, the stocker 10 has at least one robot 48 fordelivering a reticle 8 to the at least one receptacle 15 and fordelivering the reticle pod 2 to the purge nest enclosure 12. The atleast one robot 48 also retrieves the reticle 8 from the at least onereceptacle 15 and retrieves the reticle pod 2 from the purge nestenclosure 12. In some embodiments a single robot handles all of thesefunctions. In other embodiments a plurality of robots are provided, andthe robot tasks may be divided among the plurality of robots, either byfunction or by the location of the reticles 8 within the stocker 10.

The stocker further includes at least one controller 50 for controllingoperation of the robot 48 and for controlling the first gas deliveringmeans and the second gas delivering means. FIG. 5A shows two separatecontrollers 46, 50 for the stocker 10 and for the N₂ purge nest 12.Additional control functions may be performed by the same or additionalprocessors. For example, control and monitoring functions related to thework-in-progress (WIP) 6 in the fab may be performed by themanufacturing execution system (MES 52), and control and monitoringfunctions related to disposition and movement of individual pods may bemade by the material control system (MCS 54). The various controlfunctions may be physically distributed or collocated, or containedwithin a single programmed processor, or implemented in special purposehardware (application specific integrated circuitry, or ASIC).

Various functions are performed within the controller 50. The variousmeans within the controller 50 described below may correspond tosoftware modules in a one-to-one relationship, a one-to-manyrelationship, or a many-to-one relationship.

In some embodiments, the at least one controller 50 includes means forscheduling the N₂ charge operation by scheduling the at least one robot48 and the second gas delivering means for exposing the additionalreticle 42 to the second gas (N₂) within the purge nest enclosure 12.Preferably, the scheduling is based on at least one rule that determinesa duration and/or a schedule of the exposing (charging) based on afabrication process to be performed using the reticle 8.

For example, the at least one controller 50 may includes means forcontrolling the robot 48 to place the pod 2 containing a given reticle 8in the purge nest 12, and controlling the second gas delivering means todeliver N₂ to expose the additional reticle 42 to the second gas (N₂) inthe purge nest enclosure 12 for a first predetermined period of time.The first predetermined period may be 10 minutes, 15 minutes, orwhatever period is empirically determined to be suitable for purgingcontaminants from a particular type of reticle used in a particularprocess.

In some embodiments, the controller 50 includes means for scheduling theat least one robot 48 and the second gas delivering means for exposingthe additional reticle 42 to the second gas within the purge nestenclosure 12 immediately before the reticle 8 is to be delivered foruse. Then, after the reticle 8 is used in the semiconductorphotolithographic process, the operator puts the reticle pod 2 into thestocker 10 after the mask 56 is used, causing a system triggered N₂purge operation. Thus, the reticle 8 is N₂ purged shortly before andshortly after it is used in the photolithographic process. By performingthe N₂ purge operation shortly after the reticle 8 is used, the reticle8 is preserved, and the reticle 8 is also ready if it is needed foranother photolithographic process soon thereafter.

In some embodiments, the at least one controller 50 includes means forscheduling the at least one robot 48 and the second gas delivering meansfor exposing the additional reticle 42 to the second gas within thepurge nest enclosure 12 for a first predetermined period of time (e.g.,10 minutes or 15 minutes), if a second predetermined period of time(e.g., one hour or two hours) will have elapsed between a time when theadditional reticle 42 was most recently exposed to the second gas and atime when the additional reticle 42 is to be used to process a wafer(not shown). That is, in some embodiments, after an N₂ purge operationis performed on the reticle 8, the reticle 8 is ready for use within asecond predetermined period, such as two hours, after which reticle 8 isno longer ready for use and must again be subjected to an N₂ purgeoperation before it is used. When the controller 50 selects a reticle 8for use with the forecasted WIP 6, the controller also determineswhether the reticle 8 has “timed out” since it was last purged, orwhether it will time out before it is scheduled to be used. In eithercase, the controller 50 schedules another N₂ charge operation before thereticle 8 is to be used. By scheduling an N₂ purge operation when thesecond predetermined period expires, the system 4 maintains a set of oneor more reticles that are ready for immediate use when needed.

These advance purge operations may eliminate production delays thatfrequently occurred when the N₂ purging was performed manually at thelast minute before the reticles were to be used. Also, by performing thepurge operation in advance of when the operation is required, it may bepossible to avoid queueing delay due to waiting for the availability ofthe purge nest 12, in the case where many reticles are needed within ashort period of time.

In some embodiments, the at least one controller 50 includes means fordetermining when the additional reticle 42 is to be removed from thestocker 10 for use in a semiconductor fabrication process. Means areprovided for scheduling the robot 48 and the second gas delivering meansto expose the additional reticle 42 to the second gas for the firstpredetermined period of time, such that the N₂ purge operation(exposing) is completed before the time when it is desired to remove theadditional reticle 42 from the stocker 10. In order to perform thisfunction, the WIP 6 forecast by the MES 52 is used to predict when agiven lot of wafers will progress to the point in the fabricationprocess when each reticle is needed, and the reticle N₂ purge operationscan be scheduled so that the operations are completed before eachreticle is to be used.

In some embodiments, the controller 50 includes means for forecasting atype of a wafer to be processed and forecasting when that wafer is to beprocessed. Also, means are provided for selecting the additional reticle42 to be used for processing that wafer.

In some embodiments, the controller 50 is capable of operating in eithera first mode in which the controller causes the second gas deliveringmeans to deliver the second gas to the purge nest enclosure 12 duringone or more discrete periods according to a schedule or a second mode inwhich the controller 50 causes the second gas delivering means todeliver the second gas to the purge nest enclosure continuously. Thus,if the duty cycle is high, it may be desirable to operate the purge nest12 continuously in the second mode.

The exemplary stocker 10 provides a high volume density storage devicein which a reticle POD 2 will be protected by inert gas (e.g., N₂), andperforms pod charging by a sequence designed by users. The reticlestocker 10 not only saves fab space, but also improves operationproductivity of scanner and people. The reticle stocker 10 improvesoperation efficiency by N₂ charge in the reticle pod 2 in advance ofwhen the reticle 8 is needed for processing the WIP 6, to be readybefore it needed. For example in a prior system of the type shown inFIG. 1, there were 36 sets/cabinets with high area density (about 176cm2/Reticle), dispersed at various locations. A system as shown in FIG.2 provides efficient area density with a footprint of about 60cm2/Reticle, and the reticle stockers 10 may be concentrated.

The reticle stocker 10 may be implemented in new equipment.Alternatively, existing stockers may also be modified to perform the N₂purge operation. For example, an existing stocker may be modified toinclude the purge nest, the means for delivering the second gas and thecontroller. For example, an N₂ piping system and flow design canmaintain positive gas pressure within stocker. A variety of embedded N₂nest designs can be configured, depending on the size of the pod and thenumber of pods to be charged simultaneously at any given time. Thecoming WIP forecast and scheduling algorithm can be implemented as asoftware modification in a system having a controller with computerprogram code.

In some embodiments, as shown in FIG. 3, an agent based operation (ABO)function may be used for implementing a prioritization scheme. In someembodiments, the prioritization is used in combination with the WIPforecast to schedule reticles to be processed on a priority basis.

FIG. 4 is a flow chart, showing one example of steps involved in themethod of using the reticle stocker 10. The flow chart also notes wheresteps occur. Specifically, the chart notes the operation, stepsperformed at the STC 65, steps performed at the MCS 54/MES 52, actionstaken at the real time dispatcher (RTD) 60, steps occurring under agentbased operation, and actions carried out at TAP 62 (TAP is toolapplication program, whose function is to translate TCS command to theprotocol compatible with Tools.). The flow chart indicates that at step68 the POD 2 is put on the PGV inport 64, the POD ID is read at step 70,which is reported at steps 72, 74 to the MCS 54 and MES 52, in turn thePOD ID, reticle ID, and STK ID are reported to TCS 60. The informationis reported to check if the POD type was right or not, and to checkwhether the reticle needs to be charged or not. Next, verification ofthe ID at decision step 78 and verification of reticle purging atdecision step 80 are performed. After verification step 78, macro andmicro commands 82-88 are sent to dispatch the reticle to the shelf atstep 90 and the POD location and status are updated 91. Also followingthe verification of IDs at step 78, a RTD priority 92 is determined atstep 95 in the STC 65. Further, if the nest 12 is busy at step 93, themask is sent at step 94 to the purge nest 12, which causes macro andmicro commands to be issued at steps 96-104. The personal guide vehicle(PGV) port 64 goes 101 to the N2 port 101 having the ID number “portID”. The PGV port 64 is the input/output port of the reticle stockerthat can be operated manually. The commands execute a purge and updatethe POD location and status at step 106. The system also may issue analarm 73 if a read fails at step 71.

An exemplary ABO 66 system considers several factors to setup priorityand orders the MES 52/MCS 54 to execute reticle N₂ purge operations.These factors may include: (a) arrival time of coming WIP, (b) whetherthe Mask was just used or not (masks are purged as soon as possibleafter use), (c) the Mask Technology (e.g., N90, N65., etc.), and (d) theN₂ purge expired time (e.g., 2 hours), such that the N₂ purge isrepeated if the reticle is not used within the second predeterminedperiod after the most recent purge operation.

The priority system can be used in a variety of ways. For example, allN₂ purging operations for reticles that are forecast to be used within agiven period (e.g., one hour) may be assigned high priority, and anyreticle that is not forecast to be used within that given period can beassigned a lower priority. A reticle that was purged more than two hoursago, and requires another N₂ charge before use, may be assigned to a lowpriority if it is not forecast to be used within one hour. This preventsany N₂ purge operation that is performed in advance of the time thecorresponding reticle is needed from causing delay in the delivery of areticle that is to be used sooner.

Some embodiments of the method include automatically delivering thereticle pod 2 to the purge nest enclosure 12 and exposing the additionalreticle 42 to the second gas in the enclosure 12 for a firstpredetermined period of time (e.g., 15 minutes). This occurs if a secondpredetermined period of time (e.g., 2 hours) will have elapsed between atime when the additional reticle 42 was most recently exposed to thesecond gas and a time when the additional reticle 42 is scheduled to beused to process a wafer. In this situation, the additional reticle 42may be exposed to the second gas in the purge nest enclosure 12 beforethe expiration of the second predetermined period of time since the lastpurge, if the time between the current purge operation and the expectedtime of use of the reticle is less than the second predetermined period.This may eliminate queueing delays that could occur if the system wereto always wait at least the second predetermined period betweensuccessive purge operations.

In some embodiments, the method further includes determining when theadditional reticle 42 is to be removed from the stocker 10 for use in asemiconductor fabrication process. Exposure of the additional reticle 42to the second gas in the purge nest enclosure 12 is scheduled for afirst predetermined period of time. The scheduling is performed suchthat the exposing is completed before the additional reticle 42 is to beremoved from the stocker 10.

FIG. 6 is a flow chart of an exemplary method of operating the stocker10.

Step 600 includes storing at least a first reticle 8 in a housing 14 ofthe stocker 10. Preferably, a large number of reticles are stored in thehousing 14.

At step 602, a first gas (e.g., CDA) is delivered to the housing 14.

Step 604 includes forecasting a type of a wafer to be processed and whenthat wafer is to be processed. The forecasting is used for determiningwhen the additional reticle is to be removed from the stocker for use ina semiconductor fabrication process The forecasting may occur atregularly timed intervals (e.g., every one minute, or every fiveminutes), or the forecasting may occur continually. When the processingof a wafer is forecasted, the additional reticle 42 to be used forprocessing that wafer is selected.

At step 606, a determination is made whether the selected reticle is tobe used at a time that is more than a second predetermined period (e.g.,2 hours) since that reticle was most recently exposed to the purginggas. That is, the interval between the last purge and the forecastedtime of use is compared to the second predetermined period. If the timeof use will be more than the second predetermined period after the lastpurge operation for that reticle, step 608 is performed. If the timewill be less than or equal to the second predetermined period, then step616 is performed.

At step 608, automatic delivery of the reticle pod 2 to the purge nestenclosure 12 and automatic retrieval of the pod from the purge nest arescheduled.

At step 610, at least one reticle pod 2 having an additional reticle 42is automatically delivered into the purge nest enclosure 12 within thehousing 14 of the stocker 10.

At step 612, the delivery and retrieval of the reticle pod 2 anddelivery of the first gas and the second gas are controlledautomatically.

At step 614, the second gas (e.g., N₂) different from the first gas isdelivered to the purge nest enclosure 12 for the first predeterminedperiod.

At step 616, the reticle pod 2 is automatically retrieved from the purgenest enclosure 12.

At step 618, the photolithographic step is performed using the reticle.

Some embodiments include exposing the additional reticle 42 to thesecond gas for the first predetermined period of time after theadditional reticle 42 is used to process a wafer but before theadditional reticle 42 is stored in the at least one receptacle 15 of thestocker 10.

In embodiments where a second purge is performed after each use, at step620, the pod with the reticle therein is returned to the enclosure 12within the reticle stocker 10.

At step 622, the reticle is again purged of contaminants within theenclosure 12.

At step 624, the reticle is returned to one of the storage compartmentswithin the reticle stocker 12, until either the reticle times out (afterthe second predetermined period elapses), or until the reticle is againused for a photolithographic process.

Some embodiments include assigning a respective priority to eachoperation of exposing a respective additional reticle to the second gas.The scheduling step schedules for each exposing operation are based onthe respective priority of that respective exposure.

FIG. 7 is a flow chart of an exemplary method of assigning thepriorities for the purging operations. The example of FIG. 7 can be usedin a system having two priority levels (high and low) or three prioritylevels (high, medium and low).

At step 700, the respective priority of charging (exposing) a givenadditional reticle is based on at least one factor from the groupconsisting of:

(a) an amount of time until a wafer is to be processed using the givenadditional reticle; (b) a forecast of wafers to be processed within apredetermined period of time; and (c) an amount of time since the givenadditional reticle was most recently exposed to the second gas.

At step 702, a determination is made whether a given reticle to bepurged will be used in a photolithographic operation within a shortperiod of time. For example, a third predetermined period of time—e.g.,ten minutes from the time step 702 is performed—may be used as thethreshold for this determination. If the reticle is scheduled to be usedwithin the short (third predetermined) period of time, then step 703 isperformed. If the scheduled time of use will not occur for more than thethird predetermined period, then step 704 is performed.

At step 703, the reticle to be used within a short period of time isassigned the high priority.

An example in which a reticle is to be used in a short period of time isa case in which the reticle is to be used 2 hours and 10 minutes afterthe most recent purge, the purge operation requires 15 minutes, and thereticle cannot be used if it was last purged more than two hours ago. Insuch a case, the reticle must be scheduled to begin an additional purgeoperation before the two hour period since its last purge expires.Another example in which a reticle may be used within a short period oftime after step 702 is performed is a situation in which an operator hasmanually changed the queue of photolithographic processing operations tobe performed.

At step 704, a reticle that is not going to be used within the thirdpredetermined (short) period is prioritized. If the reticle was justrecently used in a photolithographic operation, then step 705 isperformed. If the reticle was not recently used in a photolithographicoperation, then step 706 is performed.

At step 705, the reticle may be assigned a high or medium priority,depending on the number of priority levels used in the system. In atwo-level (high-low) system, the recently used reticle may be assignedhigh priority, along with reticles that will be used imminently. In athree-level (high-medium-low) system, the recently used reticle may beassigned high or medium priority. Assigning the recently used reticle ahigh priority allows the reticles that will be imminently used forphotolithographic operations and the reticles that were recently used tobe purged first, before any reticles that were previously purged afterthe last use.

Alternatively, assigning the recently used reticle a medium priorityallows the reticles that will be imminently used for photolithographicoperations to be purged first, to avoid production delays, and alsoallows recently used reticles to be purged before the reticles that werepreviously purged since their last use, but have timed out since thelast purge.

In some embodiments, steps 706 and 707 may be skipped, and any reticlethat will not be used imminently, and was not recently used is assignedlow priority at step 708.

In other embodiments, at step 706, an optional third prioritization maybe made, based on the time since the last purge operation. If step 706is performed, and the time since the last purge operation exceeds thethreshold second predetermined period, step 707 is performed. If thetime since the last purge is less than the second predetermined period,step 708 is performed.

At step 707, in a two-level system, a reticle that has “timed out” (timesince the last purge is greater than the second predetermined period) isassigned the low priority. In some three-level embodiments, a reticlethat has “timed out” may be assigned medium priority, so that reticleswhich have timed out are purged before reticles that have not timed out.In other three-level embodiments, a reticle that has “timed out” may beassigned low priority, so that reticles that will be used imminently orwere recently used are purged before any other reticles.

At step 708, any reticle that will not be used imminently, was notrecently used, and has not “timed out” is assigned the low priority.

At step 710, each purge operation is scheduled based on the relativepriority of the queue of purge operations to be performed.

Although two and three level priority schemes are described above,additional one or more level(s) may be used. For example, a four levelsystem may assign priorities as follows: reticles to be used imminentlyare highest priority; recently used reticles are assigned second highestpriority; reticles that have timed out since the last purge are assignedthird highest priority, and reticles that have not timed out since thelast purge are assigned the lowest priority.

Many variations are contemplated, for example, some embodiments includeoperating the controller 50 in either a first mode in which thecontroller 50 causes the second gas delivering means to deliver thesecond gas to the purge nest enclosure 12 during one or more discreteperiods according to a schedule or a second mode in which the controller50 causes the second gas delivering means to deliver the second gas tothe purge nest enclosure 12 continuously. In some embodiments, the firstgas is clean dry air, and the second gas is nitrogen or an inert gas.

The exemplary reticle stocker 10 creates a high volume density PSMreticle stocker to fulfill the requirement of storage, specificenvironment and charging in earlier stage to avoid delays when the waferand tool are ready for processing, but the reticle 8 is not readybecause it is being charged.

The exemplary reticle stocker 10 will make sure the reticle 8 can bekept in inert gas environment and required charging before and after thephotolithographic wafer patterning process, without increasing fab spacerequirements. This stocker 10 can provide a high quality environment andhandling of MASKs to avoid wafers being scrapped due to the reticleprecipitation phenomenon and MASK repairing rate.

The stocker has potential to provide cost savings, reduce scanner idletime, and eliminate any need for a separate N₂ cabinet or N₂ Chargingtool.

The functions of the various elements shown in the figures, includingfunctional blocks labeled as “processors” or “controllers” may beprovided through the use of dedicated hardware as well as hardwarecapable of executing software in association with appropriate software.When performed by a processor, the functions may be provided by a singlededicated processor, by a single shared processor, or by a plurality ofindividual processors, some of which may be shared. Moreover, explicituse of the term “processor” or “controller” should not be construed torefer exclusively to hardware capable of executing software, and mayimplicitly include, without limitation, digital signal processor (DSP)application specific integrated circuit (ASIC) hardware, read-onlymemory (ROM) for storing software, random access memory (RAM), andnon-volatile storage. Other hardware, conventional and/or custom, mayalso be included.

The exemplary embodiments of present invention may be embodied in theform of computer-implemented processes and apparatus for practicingthose processes. Other embodiments are in the form of computer programcode embodied in tangible computer readable storage media, such asrandom access memory (RAM), floppy diskettes, read only memories (ROMs),CD-ROMs, DVD-ROMs, hard drives, flash drives, or any othercomputer-readable storage medium, wherein, when the computer programcode is loaded into and executed by a computer, the computer becomes anapparatus for practicing the invention. Other embodiments are in theform of a computer having computer program code, loaded into and/orexecuted therein, wherein the computer becomes an apparatus forpracticing the invention. When implemented on a general-purposeprocessor, the computer program code segments configure the processor tocreate specific logic circuits.

Although the invention has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the invention should beconstrued broadly, to include other variants and embodiments of theinvention, which may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the invention.

1. A method for storing and delivering a reticle comprising the stepsof: storing at least a first reticle in a housing of a stocker;delivering a first gas to the housing; automatically delivering at leastone reticle pod having an additional reticle into an enclosure having aseparate ambient environment within the housing of the stocker;delivering a second gas different from the first gas to the enclosure;automatically retrieving the reticle pod from the enclosure; andautomatically controlling delivery and retrieval of the reticle pod anddelivery of the first gas and the second gas.
 2. The method of claim 1further comprising: exposing the additional reticle to the second gas inthe enclosure for a first predetermined period of time if a secondpredetermined period of time will have elapsed between a time when theadditional reticle was most recently exposed to the second gas and atime when the additional reticle is scheduled to be used to process awafer.
 3. The method of claim 2 further comprising: determining when theadditional reticle is to be removed from the stocker for use in asemiconductor fabrication process; scheduling automatic delivery of thereticle pod to the enclosure; and exposing the additional reticle to thesecond gas in the enclosure for a first predetermined period of time,the scheduling being performed such that the exposing is completedbefore the additional reticle is to be removed from the stocker.
 4. Themethod of claim 3 further comprising: forecasting a type of a wafer tobe processed and when that wafer is to be processed; and selecting theadditional reticle to be used for processing that wafer.
 5. The methodof claim 3 further comprising: scheduling automatic delivery of thereticle pod to the enclosure; and assigning a respective priority toeach operation of exposing a respective additional reticle to the secondgas, wherein the scheduling step schedules each exposing operation basedon the respective priority of that respective exposure.
 6. The method ofclaim 5, wherein the respective priority of exposing a given additionalreticle is based on at least one factor from the group consisting of anamount of time until a wafer is to be processed using the givenadditional reticle, a forecast of wafers to be processed within apredetermined period of time, and an amount of time since the givenadditional reticle was most recently exposed to the second gas.
 7. Themethod of claim 2 further comprising exposing the additional reticle tothe second gas for a first predetermined period of time after theadditional reticle is used to process a wafer but before the additionalreticle is stored in the at least one receptacle of the stocker.
 8. Themethod of claim 1 further comprising operating a controller in a firstmode in which the controller causes the second gas delivering means todeliver the second gas to the enclosure during one or more discreteperiods according to a schedule.
 9. The method of claim 8 furthercomprising operating a controller in a second mode in which thecontroller causes the second gas delivering means to deliver the secondgas to the enclosure continuously.
 10. The method of claim 1 wherein thefirst gas is clean dry air.
 11. The method of claim 1 wherein the secondgas is selected from the group consisting of nitrogen and an inert gas.12. A stocker for storing semiconductor fabrication reticles,comprising: a stocker housing; at least one receptacle within thestocker housing for storing at least one reticle; means for delivering afirst gas to the at least one receptacle; a enclosure within the stockerhousing for receiving at least one reticle pod having an additionalreticle therein; means for delivering a second gas to the enclosure; atleast one robot for delivering a reticle to the at least one receptacleand for delivering the reticle pod to the enclosure, and for retrievingthe reticle from the at least one receptacle and the reticle pod fromthe enclosure; and at least one controller for controlling operation ofthe robot and for controlling the first gas delivering means and thesecond gas delivering means.
 13. The stocker of claim 12 wherein the atleast one controller includes means for scheduling the at least onerobot and the second gas delivering means for exposing an additionalreticle to the second gas within the enclosure based on at least onerule that determines a duration and/or a schedule of the exposing basedon a fabrication process to be performed using the reticle.
 14. Thestocker of claim 12 wherein the at least one controller includes meansfor controlling the robot and the second gas delivering means to exposethe additional reticle to the second gas in the enclosure for a firstpredetermined period of time.
 15. The stocker of claim 15 wherein the atleast one controller includes means for scheduling the at least onerobot and the second gas delivering means for exposing the additionalreticle to the second gas within the enclosure for a first predeterminedperiod of time if a second predetermined period of time will haveelapsed between a time when the additional reticle was most recentlyexposed to the second gas and a time when the additional reticle is tobe used to process a wafer.
 16. The stocker of claim 12 wherein the atleast one controller include: means for determining when the additionalreticle is to be removed from the stocker for use in a semiconductorfabrication process; and means for scheduling the robot and the secondgas delivering means to expose the additional reticle to the second gasfor the first predetermined period of time, such that the exposing iscompleted before the additional reticle is to be removed from thestocker.
 17. The stocker of claim 16 wherein the determining meansinclude: means for forecasting a type of a wafer to be processed andwhen that wafer is to be processed; and means for selecting theadditional reticle to be used for processing that wafer.
 18. The stockerof claim 12 wherein the scheduling means includes means for forecastingwhether a second predetermined period of time will have elapsed betweena time when the additional reticle was most recently exposed to thesecond gas and a time when the wafer is to be processed, and thescheduling means is responsive to the forecasting means for schedulingthe robot and the second gas delivering means to expose the reticle tothe second gas for the first predetermined period of time if the secondpredetermined period of time will have elapsed by the time when thewafer is to be processed.
 19. The stocker of claim 12 wherein the atleast one controller include means for scheduling the robot and thesecond gas delivering means to expose the additional reticle to thesecond gas for a first predetermined period of time after the additionalreticle is used to process a wafer but before the additional reticle isstored in the at least one receptacle of the stocker.
 20. The stocker ofclaim 12 wherein the controller is capable of operating in either: afirst mode in which the controller causes the second gas deliveringmeans to deliver the second gas to the enclosure during one or morediscrete periods according to a schedule; or a second mode in which thecontroller causes the second gas delivering means to deliver the secondgas to the enclosure continuously.
 21. A computer readable storagemedium having computer program code stored therein, such that, when thecomputer program code is loaded into and executed by a processor, theprocessor controls a reticle stocker to perform a method comprising thesteps of: storing at least a first reticle in a housing of a stocker;delivering a first gas to the housing; automatically delivering at leastone reticle pod having an additional reticle into an enclosure having aseparate ambient environment within the housing of the stocker;delivering a second gas different from the first gas to the enclosure;automatically retrieving the reticle pod from the enclosure; andautomatically controlling delivery and retrieval of the reticle pod anddelivery of the first gas and the second gas.